Telstra Works to Fix Secondary Issue Affecting Emergency Calls After Major Outage

Telstra says work is ongoing to resolve a "secondary issue" impacting customers trying to make calls on its network, including to Triple Zero, but some people may still be unable to contact the emergency service.

Australia's largest telecommunications company confirmed in a statement on Thursday that the frequency of the Triple Zero call error on its network had been reduced by approximately 90 per cent.

The update comes after the Telstra network suffered a major nationwide outage that left potentially millions of Australians unable to make phone calls or access the internet.

The company said in another update released after 9pm on Wednesday that while it had fixed the initial problem that sparked the outage, the secondary issue was still impacting emergency calls.

"In these cases, when you call Triple Zero you will receive an error message, and your phone will try to connect to an alternative mobile network," that update said.

"As part of our backup process, we will complete a welfare check where we detect a failed Triple Zero call."

The telco suggested customers attempting to call Triple Zero either wait for their device to connect to an alternate network or use another phone. It also encouraged customers to restart their device if the connection problems persisted.

Telstra's chief financial officer Michael Ackland told reporters on Wednesday that there was no evidence to suggest "malicious activity" might have sparked the outage.

He said he believed malfunctioning "nodes" that managed time synchronisation within some of the network data centres were to blame for the initial outage.

The nodes keep time across Telstra's entire mobile network, meaning that disruption to that service is replicated throughout the system.

"You can imagine that lots of computer systems have to synchronise time," Mr Ackland said.

"It's one of the ways that you authenticate what's going on in the network.

"And the time synchronisation in those nodes wasn't working as it should. We don't know why yet."

Telstra said that as of 5:30pm on Wednesday, it had conducted 395 welfare checks on people who were unable to make Triple Zero calls.

Of those, 85 people were referred by Telstra to emergency services for an additional welfare check or further help.

"The volume of welfare checks was higher than we expected, which prompted us to investigate further," Telstra said.

"In some cases, a phone wasn't able to connect correctly on its first call attempt, which triggered a callback from us. In other cases, the call was picked up by the TPG or Optus networks and connected successfully.

"We expect these numbers are inflated by a larger number of people calling to test Triple Zero, and as a flow-on from the core issue. Our backup systems worked as they should throughout."

In the wake of the outage, telecommunications experts and advocates have demanded changes to how companies such as Telstra are regulated in Australia so they are held accountable for mass outages.

Communications Minister Anika Wells said on Wednesday that the Australian Communications and Media Authority would conduct an investigation into the outage, which would include asking further questions of Telstra about how the incident unfolded.

Some transport shutdowns linger

The outage continues to impact Victoria's regional rail network, which came to a standstill on Wednesday, and "very limited" replacement bus services have been organised, the network operator says.

V/Line confirmed that peak morning services would be affected on Thursday while work was underway to restore the network's communications system.

The operator encouraged commuters not to travel via V/Line services if possible.

Chief executive William Tieppo told reporters on Thursday that repair works on network radio systems were ongoing, but said he was hopeful services would be restored for the afternoon peak.

"We're now in the process of undertaking testing for each of the train units that we've got here at Melbourne," he said.

"ARTC have undertaken 33 tests already on existing national trains, and that's proven to be successful."

In New South Wales on Wednesday, the outage shut down connections between trains and safety control centres and services were suspended.

The outage continues to impact the Australian Rail Track Corporation's 9,600-kilometre network, which includes passenger services on the Southern Highlands Line and the Hunter Line.

Sydney's metropolitan services do not rely on the ARTC's 4G network and were not impacted by the Telstra outage.

"The major Telstra outage continues to impact the Australian Rail Track Corporation's (ARTC) National Train Communications System (NTCS)," a spokesperson said in a statement on Thursday.

"Rail services are progressively returning to operation across affected networks following confirmation that safety-critical communications can be reliably established.

"The NTCS is a safety-critical system used across Australia's interstate and regional rail network. It enables network control centres to communicate directly with train drivers through an in-cab radio system and operates over Telstra's 4G network.

"The nationwide Telstra outage disrupted the NTCS, preventing ARTC from making or receiving essential communications with train crews and frontline workers, posing a safety risk.

"Telstra and ARTC are working around the clock to restore the communications system and safely return services to operation … Given the size and complexity of Australia's national rail network, recovery will take time and services will resume in a controlled and phased manner as communications are confirmed and all operational safety checks are completed."